A disk subsystem for storing data in a digital data processing system includes a rotatable disk which has a magnetic media on its surface and a transducer, or head, mounted on a movable arm. The arm positions a head assembly, which includes the head, a suspension, and other elements, near the surface of the rotating disk to enable magnetic transitions, representing data, to be recorded in, or read from, the magnetic media. The disk is divided into a plurality of tracks and sectors, with the tracks being concentric annular portions of the disk which are situated at selected radial distances from the disk's rotational center, and the sectors representing diverse angular sections of the disk's surface. To read data from, or write data onto, a particular track and sector, the arm moves the head assembly to a desired track, and the disk is rotated until the desired sector moves proximate the head. To write data onto the disk, the head is magnetized by a varying electrical current while the disk rotates to enable magnetic transitions to be recorded in the magnetic media. The pattern of transitions in the magnetic field recorded on the disk reflects the data which is being stored in the subsystem. To read the previously-recorded data from a particular track and sector, the arm moves the head assembly to the track and the disk rotates until the sector is proximate the head. The head senses the previously-recorded magnetic transitions and generates, in response thereto, a varying electrical signal. Other circuitry receives the signal and converts it to a digital data signal.
In modern disk subsystems, a head typically does not actually ride on the surface of the rotating disk, but instead is separated away from it a short distance. As a result of rotation of the disk, a boundary layer of air becomes entrained with it. In the head assembly, the head is typically mounted in a "slider" which serves as a mechanical support for the head and the electrical leads which carry electrical signals to and from the head. The slider is aerodynamically shaped so that the head effectively rides, or flies, on the air boundary layer. To regulate the distance between the head and disk, a continuous biasing force is applied to the arm which enables it to urge the head toward the disk surface. Typically, mechanical biasing members, such as springs, provide the biasing force.
Since the boundary layer which supports the head is generated by the rotation of the disk, it dissipates when the disk stops rotating. Two options are available to ensure that the head does not crash into the portion of the disk surface in which data is recorded, which can result in loss or corruption of the data. In one option, the head may be moved to a portion of the disk which is not used for recording of the data, typically to a portion interior of the innermost track. As the boundary layer dissipates, the biasing member forces the head to land on the disk surface. It will be appreciated, however, that when the disk is started up again, the disk surface can abrade the head, which, over time, can result in loss of performance. In addition, the friction between the disk and the head increases the torque requirements for the motor that rotates the disk, at least during start-up.
In the other option, the head is typically moved beyond the rim of the disk, or "unloaded", as the disk stops rotating, and is returned, or "loaded", when the drive is restarted. Typically, the am supporting the head is cantilevered from an actuator motor. During of unloading operation, the arm is moved away from the disk by a ramp, which forms part of the arm, that moves over a cam when the actuator moves the arm beyond the periphery of the disk. During the loading operation, the ramp tracks over the cam to position the arm so that the head is properly positioned over the disk.
Several problems arise with the use of the ramp and cam arrangement. First, the ramp and cam mechanism must be precisely fabricated so that the arm is properly lifted and maintained at a desired distance away from the disk surface after the head is unloaded. Furthermore, the ramp and cam also add to the torque requirements of the actuator motor during loading. Moreover, during loading of the head, there is an additional force, which is caused by the biasing spring, on the ramp and cam as the arm moves from the unloaded position to the loaded position. To ensure that the arm maintains an optimum loading speed so to minimize the possibility of overshooting the initial load position, some disk drives employ tachometers to monitor and control the velocity of the arm.
As noted above, during operation a head is separated from the disk by, essentially flying in the boundary layer of air entrained with the disk as it rotates. The force provided by the entrained air urges the head away from the disk, in opposition to the biasing force provided by the spring biasing member. The force provided by the entrained air varies with the speed of the air over the slider, which, in turn, increases with increasing distance from the rotational center of the disk. However, since the force which biases the head toward the disk is constant, the separation between the head and disk also varies with the radial distance from the disk's rotational center.
To help reduce flying height variations, some manufacturers have developed a head which includes complex contours which compensate for the radial variations in force provided by the air entrained by the disk. In such a head, the slider typically has two skids on its side proximate the disk formed parallel to the direction of disk rotation. A slightly recessed rail spans the skids to form, with the skids, an "H" configuration, to in turn form two recessed pockets on either side of the rail. A region of relative negative pressure is formed in the pocket at the trailing end of the slider, which is caused by the accelerated passage of air across the spanning cross-rail. The cross-rail essentially forms an air foil to provide a third force biasing the head toward the disk surface. Like the biasing force which forces the head away from the disk surface, this biasing force varies with increasing air speed, but it urges the head toward the disk surface. While this does help reduce variations in flying height at differing radial positions, such heads are difficult and expensive to fabricate. Furthermore, these features do nothing to minimize loading and unloading forces which are also required for such heads.